The present invention relates to injection molding machines and to an improved clamp mechanism for use therein.
Injection molding machines having hydraulically actuated and clamped platens are known in the art. Typically, these machines work on the principle of two separate hydraulic circuits, one to open and close the clamp and the second to generate the clamping force between the closed platens.
Some prior art machines employ a system wherein a column attached to a moving platen is blocked against the clamp piston by shutters. U.S. Pat. Nos. 4,017,236 to Penkman and 4,230,442 to Rees illustrate such a system. A gripper bushing system for blocking the column against the clamp piston is shown in co-pending U.S. patent application Ser. No. 637,814, filed Jan. 7, 1991 to Ing, now U.S. Pat. No. 5,133,655. U.S. Pat. No. 4,867,938 to Schad uses a gripper bushing system to control alternative movement of platens so that the two molding stations in the injection molding system are operated sequentially. Both these systems suffer from disadvantages which affect the performance of the clamp mechanism.
The shutter type of clamp must wait for the shutters to be interposed between the column and clamp piston before the clamp can be energized. This finite period of time during both clamp up and unclamp extends the molding cycle. Typically, shutter speed is about 0.5 seconds in each direction. As a result, approximately 1 second of the molding cycle time is waiting for shutter actuation.
The gripper style of clamping mechanism requires very high oil pressures to be generated in order to actuate the gripping bushing. Typically, 7-8,000 psi oil pressure must be used, whereas all other machine hydraulic requirements are typically 2,200 psi. This higher pressure requirement adds cost to the machine.
Additionally, gripper devices suffer from premature wearing of bushing and column surfaces. Still further, close tolerances between the bushing and the column are required. This adds cost to the clamp construction and to its maintenance.
Very large clamp mechanisms, more than 1,000 tons, mean that clamp blocking shutters or gripping bushings become very large and their disadvantages become more than proportionally counterproductive.
Accordingly, it is an object of the present invention to provide an improved clamping mechanism which reduces cycle time by decreasing clamp wait time.
It is a further object of the present invention to provide a clamping mechanism as above which operates at normal hydraulic pressures and clearances.
It is yet a further object of the present invention to provide a clamping mechanism as above whose capacity can be increased simply and without incurring major cost or cycle penalties.
It is yet a further object of the present invention to provide a clamp mechanism as above which may be used on a wide variety of machines including but not limited to injection molding machines and vertical presses.
Still another object of the present invention is to eliminate the need for an independent mold shutheight adjustment means.
Other objects and advantages of the present invention will become clearer from the following description and drawings.